Anal

Anal refers to the anus, the opening at the end of the digestive tract through which stool is excreted. In medical contexts, anal describes anatomy, physiology, and clinical conditions of this region.

Anatomy: The anal canal is about 2 to 4 cm long, located between the rectum and the perianal skin. It contains two sphincters: an internal anal sphincter (involuntary smooth muscle) and an external anal sphincter (voluntary striated muscle). The puborectalis portion of the levator ani helps form the anorectal angle and aids continence. The mucosa lines the canal, transitioning to skin at the anal verge. Blood supply comes from the superior rectal artery (a branch of the inferior mesenteric artery) and the middle and inferior rectal arteries (from the internal iliac arteries). Innervation includes autonomic nerves and the somatic pudendal nerve to the external sphincter. Lymphatic drainage goes to internal iliac nodes.

Function: The anal canal and its sphincters regulate continence and defecation. Sensation in the canal allows detection of stool and the urge to defecate. The internal sphincter maintains baseline tone, while the external sphincter provides voluntary control for withholding stool.

Clinical relevance: Common conditions include hemorrhoids (dilated anal cushions), anal fissures (painful mucosal tears), abscesses and fistulas, and less commonly anal cancer (often linked to HPV). Evaluation often involves anoscopy or other proctologic examinations. Treatments range from lifestyle and topical therapies to surgical interventions for sphincter or fissure issues.

Development: The upper anal canal originates from endoderm of the hindgut, while the lower canal arises from ectoderm; the pectinate line marks this transition.